11.1 Formulae, Functional Groups and Terminology

1. Overview

Organic chemistry is the study of carbon-based compounds. Because there are millions of these compounds, they are organized into families called homologous series based on their structures and chemical behaviors, allowing chemists to predict how new molecules will react.

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Key Definitions

• Functional Group: An atom or group of atoms that determine the chemical properties of a homologous series.
• Homologous Series: A family of similar compounds with similar chemical properties due to the presence of the same functional group.
• Saturated Compound: A molecule in which all carbon-carbon bonds are single bonds (C–C).
• Unsaturated Compound: A molecule in which one or more carbon-carbon bonds are not single bonds (e.g., C=C double bonds).
• Structural Isomers (Extended): Compounds with the same molecular formula but different structural formulae.
• Displayed Formula: A graphical representation showing every atom and every bond in a molecule.
• Structural Formula (Extended): An unambiguous description of the way atoms are arranged in a molecule without showing all individual bonds.

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Core Content

Displayed Formulae

When drawing a displayed formula, you must show all atoms and all bonds. Carbon atoms must always have 4 bonds.

• 📊Methane showing a central C with four single lines connected to four H atoms
• 📊Ethene showing two C atoms connected by a double line, each with two H atoms attached

General Formulae

Each homologous series follows a specific mathematical pattern for the number of atoms present:

Homologous Series	General Formula	Example (n=2)	Name
Alkanes	$C_nH_{2n+2}$	$C_2H_6$	Ethane
Alkenes	$C_nH_{2n}$	$C_2H_4$	Ethene
Alcohols	$C_nH_{2n+1}OH$	$C_2H_5OH$	Ethanol
Carboxylic Acids	$C_nH_{2n+1}COOH$	$CH_3COOH$	Ethanoic Acid*

*Note: In carboxylic acids, the 'n' in the formula refers to the carbons in the alkyl chain. For Ethanoic acid (2 carbons total), n=1.

Saturated vs. Unsaturated

• Saturated: Alkanes are saturated. They contain only C–C single bonds. They are generally less reactive.
• Unsaturated: Alkenes are unsaturated because they contain a C=C double bond. This double bond makes them more reactive than alkanes.

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Extended Content (Extended Only)

Characteristics of a Homologous Series

A homologous series is defined by five main characteristics:

1. Same Functional Group: e.g., all alcohols have the $-\text{OH}$ group.
2. Same General Formula: e.g., all alkanes fit $C_nH_{2n+2}$.
3. Difference of a $-\text{CH}_2-$ Unit: Each member differs from the next by one carbon and two hydrogen atoms.
4. Trend in Physical Properties: As the chain length increases, boiling points increase due to stronger intermolecular forces.
5. Similar Chemical Properties: Members undergo the same types of reactions (e.g., all alkenes undergo addition reactions).

Structural Formulae

This provides a "map" of the molecule in text form.

• Ethane: $CH_3CH_3$
• Ethene: $CH_2=CH_2$
• Ethanol: $CH_3CH_2OH$
• Methyl ethanoate (ester): $CH_3COOCH_3$

Structural Isomers

Isomers have the same atoms but different arrangements.

• Isomers of $C_4H_{10}$ (Butane):
  1. Butane: $CH_3CH_2CH_2CH_3$ (Straight chain)
  2. Methylpropane: $CH_3CH(CH_3)CH_3$ (Branched chain)
• Isomers of $C_4H_8$ (Butene):
  1. But-1-ene: $CH_3CH_2CH=CH_2$ (Double bond on the 1st carbon)
  2. But-2-ene: $CH_3CH=CHCH_3$ (Double bond on the 2nd carbon)

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Key Equations

General Formulae Symbols:

• $C$: Carbon atom
• $H$: Hydrogen atom
• $n$: Number of carbon atoms
• $O$: Oxygen atom

Combustion of Alkanes (Example: Methane):

• Word Equation: $\text{methane} + \text{oxygen} \rightarrow \text{carbon dioxide} + \text{water}$
• Symbol Equation: $CH_4(g) + 2O_2(g) \rightarrow CO_2(g) + 2H_2O(l)$

Combustion of Alcohols (Example: Ethanol):

• Word Equation: $\text{ethanol} + \text{oxygen} \rightarrow \text{carbon dioxide} + \text{water}$
• Symbol Equation: $C_2H_5OH(l) + 3O_2(g) \rightarrow 2CO_2(g) + 3H_2O(l)$

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Common Mistakes to Avoid

• ❌ Wrong: Drawing a displayed formula for an alcohol as $CH_3CH_2OH$.
• ✅ Right: In a displayed formula, you must show the bond between the O and the H ($\text{—O—H}$).
• ❌ Wrong: Giving Carbon 3 or 5 bonds.
• ✅ Right: Always count to ensure every Carbon atom has exactly four lines (bonds) coming off it.
• ❌ Wrong: Forgetting that the Carbon in the $-\text{COOH}$ group counts towards the total number of carbons in the name.
• ✅ Right: Ethanoic acid has 2 carbons total ($CH_3COOH$), not 3.

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Exam Tips

• Command Words: If the question asks to "Draw the displayed formula," use lines for all bonds. If it asks for the "structural formula," use the $CH_3CH_2...$ notation.
• Identify the Series: Look for the functional group first. If you see $C=C$, it is an alkene. If you see $-\text{OH}$, it is an alcohol.
• Isomer Questions: When asked to draw isomers, try moving a functional group (like the double bond) or creating a "branch" by taking a carbon off the end and sticking it in the middle of the chain.
• State Symbols: Organic compounds are usually gases ($C_1$ to $C_4$ alkanes) or liquids at room temperature. Combustion always produces $CO_2(g)$ and $H_2O(l)$ (under standard conditions).